Recently, there are growing demands for bulky and light paper from the viewpoint of resource saving or physical distribution cost reduction and addition of high values such as quality appearance or massive appearance. Previously, various methods for improving bulk have been attempted.
For example, the following methods have been proposed: (1) using crosslinked pulp (JPA No. Hei 4-185791 (patent document 1), JPA No. Hei 4-202895 (patent document 2), etc.), (2) mixing synthetic fibers into pulp (JPA No. Hei 3-269199 (patent document 3), etc.), (3) filling inorganic materials between pulp fibers (JPA No. Hei 3-124895 (patent document 4), etc.), (4) adding void-inducing foaming particles (JPA No. Hei 5-230798 (patent document 5), etc.), (5) adding lightly beaten pulp fibers (JPA No. Sho 58-24000 (patent document 6), etc.), (6) including a soft calendering process (JPA No. Hei 4-370293 (patent document 7), etc.), (7) adding bulking chemicals (JPA No. Hei 11-350380 (patent document 8), etc.), (8) mercerization of pulp (JPA No. Hei 7-189168 (patent document 9), etc.), (9) enzymatic treatment of pulp (JPA No. Hei 7-54293 (patent document 10), etc.), etc.
However, these methods had disadvantages such as failure to recycle pulp; a significant decrease in paper strength or stiffness due to the inhibition of bonding between fibers; an unavoidable cost increase due to the addition of different types of chemicals or fillers to pulp; inevitable fresh problems including increased foams or sizing loss during papermaking processes, etc.
According to a book of Oe et al. (non-patent document 1), beating and refining are defined as a mechanical treatment of pulp performed by passing a pulp suspension through a relatively narrow gap between a rotor and a stator, the former rotating and the latter stationary in the presence of water.
Methods for the mechanical treatment include using equipments having a metal blade or edge such as Hollander beaters, conical refiners (Jordan, Claflin, Conflo, etc.), single and double disc refiners, etc., as shown in a book edited by Paulapuro (non-patent document 2).
As shown by the literature above, it is known that the characteristics of fibers beaten by these equipments are strongly influenced by the pulp consistency during the treatment.
When pulp is treated at high consistency (30-35% by weight), the fiber length does not significantly decrease by fiber breakage, but the resulting fibers contain high proportions of flexing of fibers called curl or bending of fibers called kink so that they have a low bonding ability. When pulp is treated at low consistency (2-6% by weight), however, flexing of fibers is reduced and internal fibrillation is promoted so that the resulting fibers have a high bonding ability and sheet strength is improved, but the bulk decreases. When pulp is treated at medium consistency (10-20% by weight), the resulting fibers have intermediate properties.